The classical way to measure semiconductor and thermoelectrics alike has been to apply a voltage to “force current” through material while measuring voltage drop over a distance through a cross section to determine bulk resistance. The semiconductor industry was founded using V/I, 4-point probe methods to determine material resistance. From a determination of R, it is simple to mathematically determine bulk material resistivity based on resistance and physical dimensions. From Ohm's law, we know that I=V/R, R=V/I, R=resistance, V=voltage, and I=current. R=ρl/A, where ρ=resistivity, l=length, and A=area. Using the same equation, and with careful attention to units;
ρ=R A/l, so given the measured R and the dimensions of the material under the probes, the resistivity ρ (in Ohm-cm) can be easily determined. This methodology has been the gold standard for R and ρ determination for germanium, silicon, and some III-V compounds in semiconductor technology. However, this method has been found to be in error when thermoelectric materials are measured. Measurement values for thermoelectric material have been found to be off by a factor of 2 and more on the high side.
There are problems associated with thermoelectric material measurement. Applying voltage to material under test seems to cause the material to develop a counter voltage that inhibits current flow in thermoelectric material thus changing the expected measurement. Some investigators have suggested pulse methods for determining resistivity measurements, thinking that measurements taken with very short voltage pulses might get around the material disturbance. Errors are found using direct current, pulse, and low and high frequency alternating current methods of measure.
The following disclosure details a method that accurately predicts satisfactory the performance of components as well as the final properties of thermoelectric generators and chillers. A very simple measuring method that predicts exactly the performance of thermoelectric devices is based on operating the thermoelectric material under conditions that simulate actual solid state generator and chiller operation. With thermoelectric material tests under high current operation, the negative resistance characteristics of the material can be accurately determined. Negative resistance was found to have the same V/I slope regardless of the temperature difference, ΔT, and the voltage current ratio, V/I that was measured. Once ΔT is established in material when measuring negative resistance of thermoelectric material connected to large mass copper heat source and sink, changes in ΔT occur very slowly, allowing time to make accurate determination of current, and reduced negative voltage caused by the current flow. From measurements of −V1, the voltage when the current, I=0 and −V2, the voltage when current is allowed to flow, and the current, I(at −V2), the current at voltage V2 still at temperature ΔT, the negative resistance slope can be easily determined, including the zero-voltage-crossing for material with a particular ΔT. By forcing current to flow through thermoelectric material with ΔT induced, accurate in-process material measurements can be made. In-process material measurements are absolutely essential to successful, low cost manufacture of solid state generators and chillers, because completed solid state generators and chillers are composed of many elements and ohmic connections. Any element found faulty in assembly can be exchanged for one that performs to specification.
Accurate measurements of thermoelectric material can be made by using the forcing current method to determine negative resistance for heated junctions, with or without metal contacts. Thermoelectric device performance can be measured accurately in ingot, wafer, coupon, and ring form, before and after contact bonding. This method can be used during all stages of assembly and final test, even used for non-destructive evaluation of returned defective product. Because thermoelectric products consist of an assembly of large numbers of elements, numbering into the hundreds and thousands, one faulty or misplaced part can render a sophisticated, high performance solid state generator or chiller useless. This measuring invention has the ability to accurately test elements of the product during assembly, as the product moves along the assembly line, and this makes all the difference in having all products shippable at final test, otherwise realize a less than 10% final test yield.
This invention relates to a measuring system for determining negative resistance in thermoelectric material under thermal operation. Negative voltage measurements made on thermoelectric material while large current is caused to flow along with heat, is different than negative voltage measured with the same heat flow but without current flow. When the current flow through the thermoelectric material is increased until the negative voltage produced by a certain ΔT is reduced to zero volts, the current through the material is at a maximum for the ΔT of the device under test. Zero crossing current for the material under test can be increased by increasing ΔT, but the −V/I slope representing negative resistance of the material will stay the same as long as current does not exceed 1,000 Ampere per square centimeter for metal coated contacts. If current through the material is further increased, the voltage becomes zero and then a portion of the current flows in the positive resistance region. With make-before-break, shorted ring thermoelectric generator and chiller devices, the thermoelectric driven ring operates in the negative resistance region and the up-converter and primary portion of the circuit operates in the positive resistance region, forcing current and current drag being equal, or −V=+V. This means −V measured across the thermoelectric ring will be driving current and +V measured across the make-before break switch part of the secondary is dragging with the same polarity as a resistor with current flowing in the same direction.
Thermoelectric devices have been used for many years for specific applications where the simplicity of design warrants their use despite a low energy conversion efficiency.
Resistance and resistivity measurements of thermoelectric material are difficult to make because the act of making measurements tends to disturb the material in such a way as to render measured results questionable. When current passes through bulk thermoelectric material, the current also drags heat with the current and this disturbs any voltage readings.
This invention uses heat flow defined by the measuring system, ΔT determined by the physical dimensions of the device under test and the heat input and heat removed from the device by heat sink. The current passing through the device under test is limited to less than the thermoelectric device could produce on its own if it were short circuited by a super conducting wire. This way, the device operates with heat flow and current, the negative drive voltage at current being less negative than open circuit. Using only this measured data, all electrical parameters can be determined. The measuring technique is simple and accurate, allowing in-process use throughout the assembly, final test and even provides a non-destructive means to analyze customer returns.
Previous to this invention, methods of measure for thermoelectric material included the “Harman Method”, as in T. C. Harman, J. H. Chan, and M. J. Logan, J. Appl. Phys. 30, 1351 (1957). These methods were previously considered the gold standard for thermoelectric measurements and investigation.
A survey of measuring methods was made by J. D. Hinderman, “Thermoelectric materials evaluation program”, 3-M, Saint Paul. April 1979, NASA/STI Keywords: Evaluation, Technology Assessment, thermoelectric material, thermoelectricity.
Material test methods were also outlined by H. Iwasaki, M. Koyano, Y. Yamamura, and H. Hori, School of Material Science, JAIST, Tatsunokuchi 923-1292, Ishikawa, Japan. Center for Nano Materials and Technology, JAIST, Tatsunokuchi 923-1292, Ishikawa, Japan.
It is a purpose of this invention to provide an accurate in-process evaluation method for determining the heat to electrical energy performance for thermoelectric material beginning with starting materials, carrying through to assembled and bonded devices. It is a further purpose of this invention to provide electrical evaluation for non-bonded and bonded elements that go into the fabrication of a thermoelectric device. It is a further purpose of this invention to provide electrical evaluation for non-bonded and bonded thermoelectric devices at the end points of manufacture.
It is a further purpose of this invention to provide details for manual and automated methods for assembling thermoelectric generators and chillers.
It is a further purpose of this invention to disclose unique manufacturing and assembly aspects for efficient manufacture of thermoelectric generators and chillers.